1. Field of the Invention
The present invention relates to steel products, such as plates and pipes excellent in sulfide cracking resistance and suitable as component members of vessels, reactors and line-pipes for storing, refining or transporting crude oil or gas containing hydrogen sulfide.
2. Description of the Related Art
As is well known, hydrogen induced cracking (HIC) or sulfide stress cracking (SSC), hereinafter collectively referred to as "sulfide cracking", is a serious problem with steel plates and pipes used for oil tankers, reactors and vessels, or with line-pipes and oil-country-tubular-goods (OCTG) for crude oil or gas containing hydrogen sulfide.
HIC is generated under no external stress, whereas SSC is generated under static stress. This sulfide cracking is one of the embrittling phenomena that affect steel, that is, hydrogen embrittlement occurs due to the absorption of the hydrogen produced when steel is corroded in a wet hydrogen sulfide environment.
On the basis of studies of sulfide cracking, many counter measures have been proposed:
a) The absorption of hydrogen into steel in a wet hydrogen sulfide environment is suppressed by the addition of copper, thus improving HIC and SSC resistance. PA1 b) Since HIC occurs where sharp edges of A type inclusions composed of MnS are taken as the initiation sites, the sharp edges of the inclusions are eliminated by the addition of calcium, thereby controlling the morphology of sulfides. PA1 c) The number of inclusions is reduced and the shape of sulfide is controlled by the addition of calcium (disclosed in Unexamined Japanese Patent Publication No. SHO 56-13463). PA1 d) Since susceptibility to HIC and SSC is increased by the formation of a hardened structure at the center segregation with high concentrations of manganese and phosphorus, the segregation is reduced by soaking diffusion, or the formation of the hardened structure is prevented by accelerated cooling after rolling.
With these measures, in small scale laboratory tests using small-size specimens which are immersed in what is called a NACE solution "0.5% CH.sub.3 COOH+5% NaCl, saturated with H.sub.2 S at 1 atm, 25.degree. C.", the HIC resistance could be significantly reduced.
Steel products used in the above-mentioned environments must satisfy the requirement that the crack length ratio (CLR) after being immersed in the NACE solution for 96 hrs is less than 15%, or even less than 5%.
In recent years, however, oil wells and gas wells have been developed in more severe environments. From the view point of economy, the strength of steel products becomes more important and the operational pressure increases. The service environments for steel products, especially steel pipes is becoming more hostile.
Taking the above circumstances into consideration, a full ring test is often used to evaluate HIC resistance and SSC resistance, in addition to the conventional small-size laboratory test.
The full ring test is represented by a "CAPCIS type full ring test".
As shown in FIG. 1 and FIG. 2, respectively a vertical sectional view and a top view explaining the state where stress is applied in the CAPCIS type full ring test, a short-size steel pipe (full ring pipe) is expanded from inside to apply a tensile strength by bending the inner surface. In this state, the steel pipe 1 is filled with a NACE solution 2(NACE TM-01-77 bath with 0.5% CH.sub.3 COOH+5% NaCl solution) to thus evaluate the occurrence of HIC and SSC. This method, wherein the pipe 1 includes peripheral welding 3 and supporting bars 4 are located inside the pipe and pushed apart by jack screws 5 while gas 6 (H.sub.2 S at 1 atm) is injected into the solution, is relatively simple and suitable to evaluate the actual pipe, and therefore, it tends to be widely used.
This testing method is performed in a state where the residual stress upon the pipe-making process remains. It gives a very severe evaluation compared with the conventional small-size laboratory test in which the residual stress of the specimen is almost released after cutting.
However, steel pipes having sulfide cracking resistance sufficient to accept the CAPCIS type full ring test have not been developed.